Garage doors are usually made of a number of rectangular panels hingedly connected in successive pairs. The door panels are supported and guided by rails, each having a horizontal section anchored to the ceiling of the garage. When the door is closed, all the panels are vertical; when the door is being opened, the successive panels are pulled upwardly along the rails and pivoted through 90 degrees from a vertical to a horizontal position about their interconnecting hinges. Therefore, the panels must be sufficiently strong to avoid bending, when in horizontal position. Also, the panels must have efficient weather-proofing and good thermal insulation for use in cold climates. In applicant's own prior U.S. Pat. No. 4,802,319, dated Feb. 7, 1989, and entitled: PANEL STRUCTURE FOR GARAGE DOORS AND THE LIKE, a panel structure is described, which is designed to overcome the above-noted problems. However, it has been found in practice that the assembly of the rigid metallic frame, with the overlying casing made of synthetic resin, is rather cumbersome and time-consuming since the legs of the cross-sectionally U-shaped casing have to be spread apart for the insertion of the metallic frame, since this casing, which is made of extruded sections, has to be cut at 45 degrees to fit at the corners of the panels, since there is no provision for the easy addition of sealing strips for weather-proofing the junctions of adjacent panels; and since the plastic casing has to be cut out at the hinges, in order to directly secure said hinges to the door panel.